1. Field of the Invention
The invention relates to a semiconductor device including a protection diode for preventing electrostatic breakdown and a method of manufacturing the same, in particular, a semiconductor device relating to a structure of a Zener diode connected between a power supply line and a ground line and a method of manufacturing the same.
2. Description of the Related Art
In recent years, a signal transmission speed between devices such as, for example, a personal computer, a digital camera and so on in which a semiconductor device such as a semiconductor integrated circuit is built is rapidly increasing. Furthermore, device elements or the like forming a semiconductor integrated circuit or the like are being miniaturized day by day so as to increase the speed.
Since the miniaturization of device elements or the like leads to decrease of the electrostatic breakdown resistance to static electricity, semiconductor devices having various types of protection diodes for preventing electrostatic breakdown in semiconductor integrated circuits or the like are also being produced. FIG. 6 shows a protection diode for preventing electrostatic breakdown including first protection diodes 1, second protection diodes 2 and a Zener diode TD.
The first protection diode 1 means each of protection diodes D11, D21, D31 and D41 of which the cathode is connected to a power supply line Vp. The second protection diode 2 means each of protection diodes D12, D22, D32, D42 of which the anode is connected to a ground line Vn. The first protection diode 1 and the second protection diode 2 are connected in series between the power supply line Vp and the ground line Vn.
Terminals CH1 to CH4 are connected to the connecting nodes of the first protection diodes 1 and the second protection diodes 2, respectively. The terminals CH1 to CH4 are connected to corresponding signal lines in a semiconductor integrated circuit. The Zener diode TD is connected between the power supply line Vp and the ground line Vn. Although there are the four terminals CH1 to CH4 in this figure, the number can be increased or decreased according to need.
The operation of the protection diode for preventing electrostatic breakdown having this structure will be briefly described. For example, when high positive static electricity is applied to the signal line connected to the terminal CH1, ordinarily, the static electricity flows to the ground line Vn through the first protection diode D11 in the forward bias state and the Zener diode TD in the breakdown state since the power supply line Vp has a positive potential and the ground line Vn is grounded.
Furthermore, when high negative static electricity is applied to the signal line connected to the terminal CH1, the static electricity directly flows to the ground line Vn through the second protection diode D12 in the forward bias state. When high static electricity is applied to between the power supply line Vp and the ground line Vn, too, the static electricity flows between the power and ground lines through the Zener diode TD in the breakdown state. Therefore, any static electricity mentioned above does not break the semiconductor integrated circuit or the like through the signal lines.
As the characteristics of the protection diode for preventing electrostatic breakdown, high electrostatic breakdown resistance, low parasitic capacitance, and low parasitic resistance for an instantaneous high current flow are needed. The high electrostatic breakdown resistance is achieved by increasing the size of the protection diode for preventing electrostatic breakdown so as to decrease the density of a current flowing through the PN junction.
In order to increase the signal transmission speed, it is necessary to decrease the parasitic capacitance of the protection diode for preventing electrostatic breakdown. For this, the size reduction of the protection diode for preventing electrostatic breakdown is important. In short, the electrostatic breakdown resistance and parasitic capacitance of the protection diode for preventing electrostatic breakdown are in a trade-off relation for the diode size.
In order to realize such a desired protection diode having high electrostatic breakdown resistance, low parasitic capacitance and low parasitic resistance, it is necessary to increase the area of the protection diode and also form a high resistivity epitaxial layer on a low resistivity semiconductor substrate so as to extend a depletion layer enough when a reverse bias is applied to the protection diode so as to decrease the parasitic capacitance and the parasitic resistance.
In recent years, there is an increasing demand for a protection diode for preventing electrostatic breakdown satisfying the characteristics described above that are harder than conventional. In this case, there is an increasing demand for a dedicated semiconductor device including a protection diode for preventing electrostatic breakdown as shown in FIG. 6 in order to deal with need for a semiconductor substrate of which the resistance is lower than that of an ordinary semiconductor substrate forming a semiconductor integrated circuit or the like or to flexibly deal with static electricity that is irregularly applied to signal terminals such as input and output terminals or the like between devices in which semiconductor devices are built.
An operation principle of a protection diode for preventing electrostatic breakdown is disclosed in the Japanese Patent Application Publication No. Hei 8-102518. An example of reduction of the area of a protection diode for preventing electrostatic breakdown and so on formed on an epitaxial layer in order to protect an internal circuit is disclosed in the Japanese Patent Application Publication No. Hei 6-029466 with a cross-sectional view.
In order to realize the desired first protection diode 1 and second protection diode 2 having high electrostatic breakdown resistance, low parasitic capacitance and low parasitic resistance, there is a method in which a high resistivity epitaxial layer is formed on a low resistivity semiconductor substrate. In detail, a manufacturing method in which an ordinary process of manufacturing a bipolar type integrated circuit is partially modified is used. In this case, ordinarily, like a comparison example shown in FIG. 5 that will be described below, a protection diode for preventing electrostatic breakdown is formed by an ordinary process of manufacturing a bipolar type integrated circuit in which only a photolithographic mask pattern is modified, without an extra process as much as possible.
However, in the protection diode for preventing electrostatic breakdown of the comparison example shown in FIG. 5, the Zener diode TD as a voltage limiting element through which a high current flows cannot avoid having a large occupation area on the front surface of a P+ type semiconductor substrate 1. Therefore, it is necessary to form a protection diode for preventing electrostatic breakdown that does not need the occupation area of the Zener diode TD on the front surface of the P+ type semiconductor substrate 1 and costs low in total.